1. Field of the Invention
This invention relates to an image sensor, and more particularly to an image sensor, for use in a facsimile machine, an image scanner, an optical character reader, etc., which comprises a light emitter for applying light to an objective, a condenser lens for condensing the reflected light from the objective, and a light receptor for receiving the light from the condenser lens.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows a typical conventional image sensor. The image sensor generally comprises a transparent cover (glass cover) 11 mounted on an upper portion of a frame 10, and an illumination base plate 13 mounted fixedly in the frame 10 and supported by a support strip 18. The illumination base plate 13 has a light emitter in the form of an array of light emitting devices 12 for applying light to a document or writing W as an objective placed on the transparent cover 11. Also disposed in the frame 10 is a rod-shape condenser lens 14 for condensing the light reflected from the document W and a sensor base plate 17 on which a light receptor in the form of a sensor IC 16, including at least one array of light receiving devices 15 is mounted.
In such an image sensor, the document W is brought into contact with the transparent cover 11 as conveyed by a platen roller 20 supported on an electric equipment such as a facsimile machine. The light from the light emitting devices 12 is applied to the document W on the transparent cover 11 at an angle of about 45 degrees with respect to the transparent cover 11. The applied light is reflected from the document W perpendicularly to the plane of the transparent cover 11. The reflected light is condensed by the condenser lens 14 and is then received by the sensor IC 16, which is disposed immediately under the condenser lens 14, where the light is converted into electrical signals.
As shown in FIGS. 2 and 3, the sensor IC 16 is a multitip type in which the light receiving devices 15 are arranged in the direction of scanning. In the sensor IC 16 of FIG. 2, there are provided, for example, 64 light receiving devices a1-a64 for 64 pixels. The reflective light from the document is converted into electrical signals by the light receiving devices a1-a64. The opto-electrically converted charges are inputted to a shift register 16a from electrodes E1-E64 and are then outputted as successive electrical signals.
At that time, for realizing a light receptor of both a high sensitivity and a high resolution, it is necessary to join and arrange the sensor ICs 16 in a line at high density. When dicing the sensor IC into a desired size in the Y direction, it has been a common practice to previously provide the sensor IC with a cutting margin so that the light emitting device contiguous to the cutting portion is prevented from being broken due to cracks developing from the cutting portion in the X direction. Consequently, considering the cutting portions and the joining portions of the individual sensor ICs, it has also been a common practice to define the scanning dimension of the sensor IC 16 such that the length (pixel width) of the two light emitting devices a1 and a64 at opposite ends of the sensor IC in the X direction is slightly smaller.
It is common knowledge that the distances between the individual light receiving devices 15 of the sensor IC 16 can not be made too short as its non-illustrated tight reinforcing pattern and wiring pattern are tight or dense.
Yet, if only the opposite end pixels adjacent to the cutting portion are reduced in width, the light receiving sensitivity from the opposite light receiving devices a1 and a64 will be lowered. To obtain an average sensitivity of all the light emitting devices, it is only necessary that the Y-direction pixels of the light receiving devices a1 and a64 are lengthened, thus lowering the resolution in the sub-scanning direction (Y direction) considerably. Specifically, the staggered Y-direction lengths of the light receiving devices cause some unrecognizable images so that resolution failure between the pixels (i.e. leaving part of the image blank) will occur. Consequently the pixel width of the light receiving devices at the opposite ends must be reduced. This reduction in the pixel width would, in turn, cause the irregular sensitivity so that even if a uniform quantity of reflective light strikes the entire light receptor, the image signals of the light receiving devices a1 and a64 at the opposite ends would be impaired in characteristics, compared to the image signals of the other light receiving devices a2-a63.
In the sensor IC 16 of FIG. 3, all the light receiving devices (b2-b63) except the opposite-end light receiving devices b1 and b64 are provided respectively with shield films 19 near the opposite side in the primary scanning direction (X direction) to adjust the light-receiving sensitivity of the sensor IC, thus retarding fluctuation of sensitivity of the entire light receptor. This causes the Y-direction pixel length of the light receiving device with the shield film 19 to be substantially shorter than that of the light receiving device without any shield film 19, thus lowering the sub-scanning-direction resolution of the receiving devices b2-b63. The image passing over the shield films 19 of the light receiving devices b2-b63 would be left blank at these portions to impair the inter-pixel resolution. This problem would also occur if the shield film 19 is situated near the opposite side in the sub-scanning direction (Y direction).